U.S. patent number 3,706,919 [Application Number 05/064,240] was granted by the patent office on 1972-12-19 for capacitive gauge.
This patent grant is currently assigned to ADE Corporation. Invention is credited to Robert C. Abbe.
United States Patent |
3,706,919 |
Abbe |
December 19, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
CAPACITIVE GAUGE
Abstract
A capacitive probe having at least two electrically conducting
probe tips or electrodes centrally placed within an electrically
conducting housing and axially displaced from each other within the
housing. A physical dimension is measured by determining the
capacitance between one probe tip and a surface, the capacitance
therebetween varying with the dimension being measured. Precise
machining of planar probe tips and assemblies to make them
identical, their close placement, and the use of a moisture
impenetrable dielectric for supporting the tips within the housing
insure precision measurement and environmental independence. This
precision and its maintenance is augmented by electronic excitation
circuitry for the probe tips which maintain the instantaneous
electric potential on each tip approximately equal and which gives
an output signal whose average variation from a ground or common
potential is directly indicative of the distance being gauged.
Modifications to the basic probe construction include: provisions
for guarding each probe tip with a substantially equal potential:
thin or thick film deposition probe tip constructions; and probes
having a plurality of tips for sensing a multiplicity of factors
influencing the capacitance between the probe and a surface with
circuits for separating the factors. A specific application of this
for dielectric strip width measurement is presented.
Inventors: |
Abbe; Robert C. (Newton,
MA) |
Assignee: |
ADE Corporation (Newton,
MA)
|
Family
ID: |
22054537 |
Appl.
No.: |
05/064,240 |
Filed: |
August 17, 1970 |
Current U.S.
Class: |
361/280;
361/278 |
Current CPC
Class: |
G01R
27/02 (20130101); G01B 7/023 (20130101) |
Current International
Class: |
G01B
7/02 (20060101); G01R 27/02 (20060101); H01g
007/00 () |
Field of
Search: |
;317/256,246,261,242,DIG.2 ;340/200 ;348/258C |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldberg; E. A.
Claims
What I claim is:
1. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said
housing;
d. at least one balancing electrode corresponding to each said
measuring electrode and being of substantially the same size as its
corresponding measuring electrode; and,
e. means for supporting each said balancing electrode within and
for insulating it from said housing; said means for supporting each
said balancing electrode being of substantially the same size as
said means for supporting its corresponding measuring
electrode.
2. The probe of claim 1 characterized by said means for supporting
said measuring electrode and said means for supporting said
balancing electrode being substantially impenetrable by matter from
the probe's environment so as to be adapted to maintain their
dielectric constant substantially without change against the
influences of the environment of said gauge.
3. The probe of claim 1 characterized by having the values of
capacitance between said housing and said measuring and balancing
electrodes substantially unaffected by the environment of said
probe.
4. The probe of claim 1 characterized by having the values of
capacitance between said housing and said measuring and balancing
electrodes substantially equal.
5. The probe of claim 1 wherein said measuring and balancing
electrodes and their supporting means have equal coefficients of
temperature expansion.
6. The probe of claim 1 further characterized by having means for
sealing the opening of said housing against the penetration of
matter from the environment of said probe.
7. The probe of claim 1 wherein said measuring and balancing
electrodes are spaced along the axis of said housing and away from
said housing, said electrodes being characterized as being axially
thin relative to the distance between said electrodes and said
housing.
8. The probe of claim 1 having a substantially plane surface across
the opening of said housing, said plane surface comprising an
exposed face of each said measuring electrode and an exposed face
of said means for supporting each said measuring electrode and the
termination of said housing at said opening.
9. The probe of claim 1 having a contoured surface across the
opening of said housing, said contoured surface comprising an
exposed face of each said measuring electrode, an exposed face of
said means for supporting said measuring electrode, and the
termination of said housing at said opening.
10. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive cylindrical housing having an
opening;
b. a first assembly comprising:
i. at least one measuring electrode;
ii. a dielectric first annulus for supporting said measuring
electrode within and insulating it from said housing near said
opening; and,
iii. an electrically conducting first ring, said measuring
electrode having a cylindrical shape with a periphery substantially
parallel to an inner wall of said housing, said dielectric first
annulus surrounding the periphery of said measuring electrode and
in electrical contact therewith, the periphery of said first
annulus being substantially parallel to the inner wall of said
housing, said first ring surrounding the periphery of said first
annulus and making electrical contact therewith and with said
housing; and,
c. a second assembly comprising:
i. at least one balancing electrode corresponding to each said
measuring electrode and of similar dimensions to its corresponding
measuring electrode;
ii. a dielectric second annulus for supporting said balancing
electrode within and for insulating it from said housing, and
iii. an electrically conducting second ring, said second assembly
being substantially the same in size, shape, and construction as
said first assembly.
11. The probe of claim 10 wherein each said measuring electrode,
said first ring, and said first annulus are substantially identical
to each said corresponding balancing electrode, said second ring,
and said second annulus respectively with the machining tolerances
therefore substantially better than one one-thousandth of an inch
on at least the following dimensions:
a. the peripheral shape and height of said electrodes;
b. the inner surface shape and height of said annular rings;
and
c. the overall spacing between the inner surface of said annular
rings and the outer peripheries of said electrodes through said
annuli wherein there is established equality of the capacitance
between said measuring electrode and said housing relative to the
capacitance between said balancing electrode and said housing.
12. The probe of claim 11 further characterized by said first and
second annuli being adapted to maintain their dielectric constant
substantially without change against the influences of the
environment of said probe.
13. The probe of claim 11 characterized by having said first and
second annuli impenetrable to and non-absorptive of matter from the
environmental of said probe.
14. The probe of claim 11 further characterized by having means for
sealing the opening of said housing against the penetration of
matter from the environment of said probe.
15. The probe of claim 14 wherein said means for sealing comprises
a covering over said housing at its open end and over an exposed
surface of said first assembly.
16. The probe of claim 10 further characterized by said first and
second annuli being adapted to maintain their dielectric constant
substantially without change against the influences of the
environment of said probe.
17. The probe of claim 10 further characterized by said rings,
annuli and electrodes having matched coefficients of temperature
expansion.
18. The probe of claim 10 characterized by having said first and
second annuli impenetrable to and non-absorptive of matter from the
environment of said probe.
19. The probe of claim 10 further characterized by having means for
sealing the opening of said housing against the penetration of
matter from the environment of said probe.
20. The probe of claim 10 further characterized by having a
retaining ring into which said first and second assemblies fit from
opposing openings and are retained, said housing adapted for
receiving and resiliently retaining said retaining ring.
21. The probe of claim 20 further characterized by said first and
second annuli being adapted to maintain their dielectric constant
substantially without change against the influences of the
environment of said probe.
22. The probe of claim 20 characterized by having said first and
second annuli impenetrable to and non-absorptive of matter from the
environment of said probe.
23. The probe of claim 20 further characterized by said rings,
annuli and electrodes having matched coefficients of temperature
expansion.
24. The probe of claim 20 further characterized by having means for
sealing the opening of said housing against the penetration of
matter from the environment of said probe.
25. The probe of claim 11 further characterized by said rings,
annuli and electrodes having matched coefficients of temperature
expansion.
26. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said
housing;
d. at least one balancing electrode corresponding to each said
measuring electrode and of similar dimensions to its corresponding
measuring electrode; and,
e. means for supporting each said balancing electrode within and
for insulating it from said housing, said means for supporting each
said balancing electrode being dimensioned similar to said means
for supporting its corresponding measuring electrode, said means
for supporting said measuring electrode and said means for
supporting said balancing electrode comprising a dielectric oxide
of a metal.
27. The probe of claim 26 wherein said supporting means comprise
the oxide of a metal from the group consisting of aluminum,
beryllium, silicon, and titanium.
28. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said
housing;
d. at least one balancing electrode corresponding to each said
measuring electrode;
e. means for supporting each said balancing electrode within and
for insulating it from said housing, said means for supporting each
said balancing electrode being dimensioned similar to said means
for supporting its corresponding measuring electrode; and,
f. filler means placed between each said electrode and each
supporting means, said filler means being a ductile conducting
substance.
29. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said
housing;
d. at least one balancing electrode corresponding to each said
measuring electrode and of similar dimensions to its corresponding
measuring electrode;
e. means for supporting each said balancing electrode within and
for insulating it from said housing, said means for supporting each
said balancing electrode being dimensioned similar to said means
for supporting its corresponding measuring electrode; and,
f. means for sealing the opening of said housing against foreign
matter, said sealing means comprising a cover over the opening of
said housing and comprising an oxide of metal from the group
consisting of aluminum, beryllium, silicon and titanium.
30. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said
housing;
d. at least one balancing electrode corresponding to each said
measuring electrode and of similar dimensions to its corresponding
measuring electrode;
e. means for supporting each said balancing electrode within and
for insulating it from said housing, said means for supporting each
said balancing electrode being dimensioned similar to said means
for supporting its corresponding electrode; and;
f. means for sealing the opening of said housing against foreign
matter, said sealing means comprising a covering over the opening
of said housing and comprising a silicon.
31. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said
housing;
d. at least one balancing electrode corresponding to each said
measuring electrode and of similar dimensions to its corresponding
measuring electrode;
e. means for supporting each said balancing electrode within and
for insulating it from said housing, said means for supporting each
said balancing electrode being dimensioned similar to said means
for supporting its corresponding measuring electrode; and,
f. a retaining ring into which said means for supporting said
measuring and balancing electrodes fit and are retained within said
housing, said housing being adapted to receive and resiliently
retain said retaining ring.
32. A capacitive probe for use in a dimension measuring gauge
comprising:
a. an electrically conductive housing having an opening;
b. at least one measuring electrode;
c. means for supporting each said measuring electrode within and
insulating it from said housing near the opening of said housing,
said supporting means being a dielectric first substrate with a
measuring electrode comprising a first central deposition
thereon;
d. at least one balancing electrode corresponding to each said
measuring electrode and of similar dimensions to its corresponding
measuring electrode; and
e. means for supporting each said balancing electrode within and
for insulating it from said housing, said means for supporting each
said balancing electrode being dimensioned similar to said means
for supporting its corresponding measuring electrode, said
balancing electrode supporting means being a dielectric second
substrate with a balancing electrode comprising a second central
deposition thereon, said housing being adapted to receive and
retain said first and second substrates in substantially parallel
relationship and with said central depositions being aligned along
a central axis through said housing.
33. The capacitive probe of claim 32 further comprising:
a. an electrically conducting first deposition band located on said
first substrate and surrounding said first central deposition;
and,
b. an electrically conducting second deposition band located on
said second substrate and surrounding said second central
deposition, said first and second deposition bands being adapted to
make electrical contact with said housing along an outer periphery
of said first and second substrates respectively.
34. The capacitive probe of claim 33 further characterized by said
first and second substrates being non-absorptive of and
impenetrable to matter from the environment of said probe.
35. The probe of claim 33 further characterized by having means for
sealing the interior of said housing against penetration thereinto
of matter from the environment of said probe.
36. The probe of claim 35 wherein said means for sealing is a
non-conducting covering across the opening of said housing and over
said first substrate.
37. The probe of claim 33 further characterized:
a. by said first and second central depositions being of
substantially similar configuration, size and orientation to each
other and substantially similar configuration and orientation to
the cross-sectional shape of an interior wall of said housing in a
plane perpendicular to the central axis of said housing; and,
b. by having the configuration, orientation, and size of said first
and second deposition bands substantially equal to each other with
an inner edge of said first and second deposition bands of
substantially similar configuration and orientation to an outer
edge of said first and second central depositions respectively to
maintain a constant separation between the inner edges of said
bands and outer edges of said central depositions.
38. The probe of claim 33 wherein said first and second substrates
are characterized by having low and equal temperature coefficients
of expansion.
39. The probe of claim 33 wherein said deposition bands and said
central depositions are further characterized by being as thin as
is consistant with their being good electrical conductors.
40. The probe of claim 32 wherein said first and second substrates
are characterized by having low and equal temperature coefficients
of expansion.
41. The probe of claim 32 wherein said central depositions are
further characterized by being thin in the direction of said
housing's axis.
42. The probe of claim 32 wherein said second substrate is
characterized by having a central hole passing through said second
central deposition in the direction of said housing's axis.
43. The probe of claim 42 wherein said second central deposition is
further characterized by passing through said hole in said second
substrate from one side thereof to the other and forming a
continuous deposition to electrically connect the sides of said
second substrate through said hole.
44. The capacitive probe of claim 1 characterized by having means
for guarding said measuring and balancing electrodes by surrounding
said measuring and balancing electrodes with an electrical
conductor at approximately the same instantaneous potential as is
applied to said electrodes, whereby flux switching in the flux of
each said measuring electrode from a surface being gauged to said
housing is minimized.
45. The probe of claim 44 wherein said means for guarding said
measuring and balancing electrodes comprises an inner conductor
insulatingly supported within an inner wall of said housing.
46. The probe of claim 44 characterized by having said means for
supporting impenetrable to and non-absorptive of matter from the
environment of said probe.
47. The probe of claim 44 further characterized by having means for
sealing the opening of said housing against the penetration of
matter from the environment of said probe.
48. A compensating capacitive probe for use in measuring the
capacitance between said probe and a surface over a plurality of
separate areas between said probe and said surface, said probe
comprising:
a. a housing of electrically conductive material enclosing a volume
and having at least one open end,
b. first and second tip assemblies, each assembly comprising:
i. a dielectric sheet;
ii. a plurality of electrically conducting probe tips associated
with said sheet; and,
c. means for securing each said tip assembly within said housing
with said first tip assembly placed at an open end of said housing
to define an exposed surface comprising an end section of said
housing, an exposed surface of said dielectric sheet, and an
exposed surface of each said tip associated with said first tip
assembly, and, with said second tip assembly secured substantially
parallel to said first tip assembly and further within said
housing.
49. The compensating capacitive probe of claim 48 wherein said
first and second tip assemblies are substantially identical.
50. The compensating capacitive probe of claim 48 wherein:
a. said housing further comprises an interior cylindrical wall,
rectangular in cross section with a rectangular cross-sectional
opening where said first tip assembly is secured; and,
b. a plurality of conducting rectangular tips are provided in line
across the long dimension of the rectangular cross-section of said
housing with each peripheral edge of each said tip substantially
parallel to an edge of said interior cylindrical wall.
51. The compensating capacitive probe of claim 48 further including
means for guarding said electrically conducting tips between said
housing and said tips whereby electric flux from said electrically
conducting tips in said first tip assembly is confined toward the
direction of said surface.
52. The compensating capacitive probe of claim 48 wherein said tips
fit into perforations in said dielectric sheets and are flush with
the opposing faces of said dielectric sheets.
53. The compensating capacitive probe of claim 52 wherein at least
the following dimensions occurring in both first and second tip
assemblies are maintained substantially identical for corresponding
portions of said first and second tip assemblies to tolerances of
substantially better than one one-thousandths of an inch:
a. the distances between peripheral edges of said tips and the
interior cylindrical wall of said housing;
b. the thickness of each said tip at its periphery in a direction
from one face to the opposing face of each said dielectric
sheet;
c. the dimensions of each said tip lying parallel to its face;
and
d. the degree to which faces of each said tip are parallel with the
faces of said dielectric sheets and are parallel to said
surface.
54. The compensating capacitive probe of claim 53 wherein the
thickness of each said electrically conducting tip is of low
significance in determining the capacitance between each said tip
and said housing or said surface.
55. The compensating capacitive probe of claim 48 wherein:
a. each said dielectric sheet is a dielectric substrate; and,
b. each said electrically conducting tip is a deposition of an
electrically conducting substance on said substrate.
56. The compensating capacitive probe of claim 55 further
comprising a covering over said first tip assembly and housing,
said covering inhibiting the passage of matter from the environment
of said compensating capacitive probe into the volume enclosed by
said housing.
57. The compensating capacitive probe of claim 55 wherein each said
dielectric substrate is further characterized by being
non-absorptive and impenetrable to matter from the environment of
said compensating capacitive probe.
58. The compensating capacitive probe of claim 48 further
comprising a covering over said first tip assembly and extending to
said housing, said covering adapted to inhibit the penetration of
matter from the environment of said compensating capacitive probe
into the volume bounded by said housing.
59. The compensating capacitive probe of claim 48 wherein each said
dielectric sheet is further characterized by being non-absorptive
and impenetrable to matter from the environment of said
compensating capacitive probe.
Description
BACKGROUND OF THE INVENTION
Highly accurate and automatic non-contact dimensional or distance
gauging has increasingly important industrial applications today,
particularly where variations of small fractions of an inch must be
measured.
Where metallic sheets, bearings, or other surfaces are formed to
very stringent dimensional tolerances, it is often necessary to
monitor the forming process or finished product for progress
towards or compliance with the dimensional constraints. This same
control is also necessary in the manufacture of dielectric or
non-conducting forms. Often variations in fractions of a mil or
less are significant.
While monitoring of these dimensions is possible today, it is a
costly operation particularly where large areas must be monitored.
This requires a laborious and a time consuming equipment set up;
equipment calibration, and surface investigation. What is needed by
the industry is a highly accurate automatic readout gauge which
will maintain its accuracy under varying use and environment
conditions. Such a gauge must be flexible enough to adapt itself to
use in a wide variety of applications, to make distance
measurements across small areas between surface and gauge and yet
be able to scan large areas of surface for dimensional
compliances.
It is, of course, known that the variation in capacitance between
an electrode and a surface can be used to obtain an indication of
the variation in distance between the electrode and the surface.
This art has undergone some refinement but lacked the accuracy,
flexibility and repeatability of the herein disclosed
invention.
Among the problems of the prior art designs were large stray
capacitances and thermally or environmentally induced variations in
them. These variations caused large changes in electrical output
signals of circuitry operating with the probe despite no variation
in the capacitance being sensed. The size of the prior art probes
was correspondingly limited as to minimum dimensions, making them
inappropriate for a great many applications. Additionally, prior
art approaches to probe excitation, including ordinary bridge
circuitry, caused differently varying and different instantaneous
voltages to appear on internal capacitance sensitive probe
electrodes. This type of excitation fostered electrode interactions
with accompanying instabilities that further reduced the utility of
previous designs.
When using capacitance to measure variations in distance between an
electrode and a surface or to measure variations in a capacitance
affecting substance intervening between the electrode and the
surface, variations in other parameters may affect the measured
capacitance and generate an erroneous indication. To compensate for
the error induced by the variations in these unwanted parameters,
additional electrodes can be added to measure capacitance
variations between themselves and the surface with these
capacitance variations produced in different ways by variations in
the parameters affecting those capacitances. The resulting
multiplicity of relationships between a multiplicity of variable
parameters allows their independent isolation or elimination.
The present invention was developed in part from an investigation
of the faults of earlier dual electrode capacitance sensing probes
which failed to maintain or achieve the desired accuracies in
sensing capacitance between itself and a surface.
According to this invention a dual electrode capacitive sensing
probe can be designed that overcomes the drawbacks of the prior art
probes and achieves the consistent accuracy sought
It is thus a general object of this invention to provide a
capacitive probe for sensing the capacitance between itself and a
surface which achieves a high degree of initial accuracy in
detecting this capacitance and its variations.
It is a further general object of this invention to provide a
capacitive probe for sensing the capacitance between itself and a
surface which maintains a high degree of accuracy in the
measurement of this capacitance and its variations in the face of
environmental influences.
It is a more specific object of this invention to provide a
capacitive probe for sensing the capacitance between itself and a
surface which eliminates the error inducing effect from the
presence of foreign materials or contaminants in the probe's
environment.
It is a further specific object of the present invention to provide
a capacititive probe for sensing the capacitance between itself and
a surface which probe has either a dielectric support for its
electrodes or a covering over its measuring electrode which is
impenetrable to and non-absorptive of foreign matter and
contaminants from the probe's environment.
It is a further specific object of this invention to provide a
capacitive probe for measuring the capacitance between itself and a
surface wherein excitation circuitry for the electrodes of the
probe is designed to minimize the effect of unwanted influences
from the probe's environment.
It is a further specific object of this invention to provide a
capacitive probe for measuring the capacitance between itself and a
surface where the excitation circuitry for the probe has an output
signal whose average variation from a ground or common reference is
the indicium of the varying capacitance being measured.
It is a further specific object of the present invention to provide
a capacitive probe for measuring the capacitance between itself and
a surface which may be conveniently guarded to maintain the basic
theoretical relationship between the capacitance and the distance
from the measuring electrode to the surface.
It is a further specific object of this invention to provide a
capacitive probe for measuring the capacitance between itself and a
surface, where a plurality of measuring electrodes are provided to
sense a plurality of parameters affecting the capacitance between
the probe and the surface in a manner which permits electronic
processing to separate each parameter.
BRIEF SUMMARY OF THE INVENTION
In an exemplary preferred embodiment of the present invention a
capacitive probe is shown comprising an electrically conducting
cylindrical housing enclosing measuring and balancing electrodes or
probe tips which are centrally placed within the housing and
displaced axially from each other but still close to each other
along a central axis of the housing. The measuring electrode is
placed across an open end of the housing where it can be
conveniently placed within practical capacitance sensing distance
of an electrically conducting surface for sensing the capacitance
between the measuring electrode and the surface. Each electrode is
formed substantially identically to the other in a relatively thin
plate or disc-shape. Each electrode is dielectrically supported
within the housing with precisely maintained distances through the
dielectric between the periphery of each electrode and the nearest
points of electrical conduction on the housing which may be rings
or bands extending inward from the housing toward the electrode.
Each assembly which includes an electrode, a dielectric support and
a ring or band is designed to have a homogeneous coefficient of
temperature expansion and thin shape in order to minimize and
control the variation in capacitance between the housing and each
electrode from environmental influences including temperature.
Excitation from impedance measuring circuitry for each electrode,
furthermore, maintains the instantaneous electric potential on each
electrode at approximately the same level so that each electrode
partially guards the other allowing their close placement without
interactions between the electrodes. This construction further
permits certain dimensions to be maintained to very strict
tolerances while other tolerances are less tightly maintained with
the resulting performance of the entire probe improved in
accordance with the strictness of the tight tolerances.
The level of accuracy sought for this probe has necessitated a
selection of dielectrics for supporting the electrodes which are
specially impenetrable and non-absorptive of foreign matter and
contaminants from the environment of the probe. Unless care is
taken to select a dielectric with very low absorption, minute
absorption quantities have been found capable of causing erratic
operation of the probe and unbalance in the capacitance between
housing and electrodes.
An Alternative exemplary embodiment shows each electrode formed as
an electrically conducting deposition upon a dielectric substrate
and surrounded by an electrically conducting band also on the
substrate. A hole may be used to perforate the interior or
balancing electrode within the housing to pass some of the fringe
field effecting the balancing electrode into a region where it can
affect the measuring electrode.
In a further exemplary modification, a guard ring may be provided
as either a second inner dielectrically spaced conductor on the
housing or a surrounding electrically conducting deposition on a
substrate. The guard ring is maintained at approximately the same
voltage as each electrode by the excitation from the impedance
measuring circuitry. The guard ring increases the effective dynamic
range of distance or capacitance which can be measured between the
measuring electrode and the surface, lowers the capacitances from
housing to electrodes and/or allows location of the excitation and
impedance measuring circuitry remotely from the capacitive probe
instead of housing it in a chassis directly associated with the
probe.
A multiple electrode probe modification is shown having several
sets of measuring electrodes and optionally an identical balancing
electrode located behind each measuring electrode inside a housing.
In particular where a dielectric strip is moved over a path between
the measuring electrodes and a surface and where it is desired to
sense variations in the width of the strip, three, four, or five
sets of electrodes are sued to sense three, four or five different
capacitances varying with the strip width thickness, and dielectric
constant and the spacing between the probe and surface. Each
capacitance depends on these parameters in different ways so that
processing electronics can isolate one dimension by eliminating
variations in the other dimensions' capacitances.
Exemplary impedance measuring circuitry associated with each probe
comprises an oscillator supplying alternating electric excitation
to two DC blocking impedance arms which are connected to opposite
junction points of a diode ring. The remaining two opposite
junction points between diodes are connected to measuring and
balancing electrodes respectively. AC return is through the housing
as ground to the oscillator. A DC return and indicator circuit
comprising a serially connected inductor and DC meter indicates a
DC offset representative of the unbalance in the capacitance to
ground between the measuring and balancing electrodes.
The features of this invention will be more clearly understood by
referring to exemplary preferred embodiments in the below detailed
description in conjunction with the following drawings of
which:
FIG. 1 is a vertical section and partial schematic view of a basic
form of probe and exciting ring circuit;
FIG. 2 is a vertical section and partial schematic view of a probe
and circuitry exemploary of the invention;
FIGS. 3a and 3b are cross-sectional end view of two forms of probe
suitable to the construction of FIG. 2.
FIG. 4 is a sectional view of a probe with a modified probe tip
assembly;
FIG. 5 is a sectional and partial schematic view of a modified
probe showing a complete housing and readily manufactured
insert;
FIG. 6 is a sectional view of a modified probe design using metal
deposition techniques;
FIG. 7 is a sectional view of a capacitive probe of the type
indicated in FIG. 6, but with a modified probe tip assembly;
FIG. 8 is a sectional and partial schematic view of a capacitive
probe having guarding means;
FIG. 9 is a sectional view of a partial probe having guarding means
produced by metal deposition techniques;
FIG. 10 is an alternative orientation for a probe tip assembly of
the construction of FIG. 9;
FIG. 11 is a partial block diagram and partial schematic diagram of
an impedance measuring circuit or bridge cooperating with the
capacitance of a capacitive probe;
FIG. 12 is a block diagram of circuitry used to process the output
of a capacitive probe for a direct distance indication;
FIG. 13 is a sectional view of a multiple electrode capacitive
probe used for sensing the width of a dielectric strip;
FIG. 14 is a sectional view on line 14--14 of FIG. 13;
FIG. 15 is a schematic and sectional view of a multiple electrode
capacitive probe used for sensing distance, and the width and
thickness of a dielectric strip;
FIG. 16 is a block diagram and partial schematic of processing
circuitry for the capacitive probes of FIGS. 13, 14, and 15 and
showing a plurality of terminals which can be interconnected with
each other in numerous ways.
FIGS. 17a, b, and c show alternative interconnection systems for
the terminals of FIG. 16; and,
FIG. 18 shows in block diagram and partial schematic view an
alternative processing circuit for the capacitive probe of FIGS. 13
and 14.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 there is shown in sectional and partial
schematic view a capacitive probe previously sold in the United
States comprising cylindrical electrically conductive probe tips 14
and 16 with a cylindrical dielectric spacer 22 separating them. The
tip 16 has a greater radius than the tip 14 and the spacer 22 has
the smallest radius of the three. Initially the probe tip 14 is
substantially longer in the axial direction than as shown in FIG.
1, the ultimate form for the tip. The initial combination of the
two tips are then electrically connected to opposite junctions of a
serially joined diode ring 30 via electrical leads 28 and 32
contacting tips 14 and 16 respectively. This entire sub-assembly is
inserted within an electrically conducting housing 12 and inner
dielectric sleeve 20 where they are potted in place by any of the
commonly available potting compounds. An open end 24 of the housing
12 has a region of enlarged diameter 26 to accommodate the tip 14
of greater radius. The open end 24 and the tip 14 are then machined
back until there is equality in the capacitances between the
housing 12 and the two tips 14 and 16 respectively. Two
electrically conducting leads 38 are connected to the remaining two
junction points on the ring 30, pas through the housing 12 away
from the open end 24, and are ultimately connected to additional
circuitry.
The advantages of the improved capacitive probe can be better
understood from a description of its features as shown partially in
vertical section and partially in schematic diagram in FIG. 2. As
shown there, a cylindrical electrically conductive housing or
cylinder 40 terminates to form an open end at 42. First and second
capacitive probe tip assemblies 44 are generally planar shaped and
fit within an inner wall 46 of the cylinder 40 perpendicular to an
axis of the cylinder 40. The shape of the inner wall 46 in a plane
perpendicular to the axis of the cylinder 40 can be any shape but
is normally square, rectangular, or circular as indicated by FIGS.
3A and 3B. The outer periphery edges of the assemblies 44 are of a
corresponding size and shape to allow a close fit with the inner
wall 46.
The first probe tip assembly within the inner wall 46 is located at
the open end termination 42 and closes off the end of the cylinder
40 from an inner region 48. The first probe tip assembly comprises
a first electrically conducting disc-shaped electrode or tip 50
located with the faces of the disc-shaped tip perpendicular to a
central axis 52 of the cylinder 40 and with a peripheral surface 54
of the tip 50 parallel to the axis 52 and substantially equidistant
from the interior wall 46 of the cylinder 40 at all points. Further
comprising the first probe tip assembly, a dielectric annulus 56
surrounds the periphery 54 of the tip 50 and acts as a dielectric
gap between the tip 50 and an electrically conducting ring 58 which
in turn surrounds the outer periphery of the dielectric annulus 56
and extends radially to make electrical contact with cylinder
40.
During fabrication of the first probe tip assembly a very small
amount of a ductile substance such as solder may be placed as a
filler 60 between the points of contact of the tip 50, annulus 56,
ring 58, and cylinder 40. Typically the filler 60 is only between
the ring 58 and cylinder 40 to accommodate differences in thermal
expansion and insure good electrical contact over the surfaces in
contact between these elements during stresses and strains produced
by environmental influences. Where the tip 50, annulus 56 and ring
58 are not bound by a filler 60, fusion techniques may be used to
bind them.
The second probe tip assembly is substantially identical to the
first probe tip assembly and is placed between the inner wall 46 of
the cylinder 40, inside the region 48, back from the open end 42
butting against a locating sleeve 62. A dielectric annulus 66 is
located between a ring 64 and a tip or electrode 68 of the second
assembly in identical manner to the first assembly. Filler 60 may
also be added on the points of contact between the cylinder 40,
ring 64, annulus 66 and tip 68, or only between the cylinder 40 and
ring 64.
A hole 70 perforates the tip 68 parallel to the axis 52 and allows
an electrical lead 72 to pass from a point of electrical contact
with the tip 50 to a diode matrix 74 located on the opposite side
of the second probe assembly. An electrical lead 76 electrically
connects tip 68 within the diode matrix 74. Within the diode matrix
74 diodes 78 are serially connected in the same conduction
direction to form a closed path with the electrical leads 72 and 76
connected to opposite junction points between the diodes 78. From
the other two junction points on the path electrical conduction
leads 82 and 84 exit from the diode matrix 74 to DC blocking
impedance or capacitive arms 86 and 88 respectively. At the
frequency of oscillator 90 the value of the impedance of capacitors
86 and 88 is typically,but not necessarily, at least an order of
magnitude lower than the capacitive impedance between the cylinder
40 and the tips 68 and 50. The opposite terminals of the capacitors
86 and 88 are fed in common by an oscillator 90 whose other
terminal is electrically connected to the cylinder 40 which is for
convenience at circuit ground. The oscillator 90 supplies an
alternating electric excitation which is referenced to ground.
Two indicating circuits 92 are each composed of an inductor 94 and
a meter 96 connected in series to conduct DC signals to ground from
the leads 82 and 84 respectively.
Moisture impenetrable covering 98 is alternatively placed across
open end termination 42 of the cylinder 40 and the outer surface of
the first probe tip assembly and is sealed to the cylinder 40 at
its termination 42 and optionally to the assembly 44 containing the
tips 50.
The capacitive probe as above described is used to great advantage
in measuring or gauging distances between the outer face of the tip
50 and a grounded conducting surface 100 by detecting through
either indicator 92 the difference in the capacitance to ground of
the tip 50 relative to the tip 68. For this purpose the surface 100
will normally be parallel to the outer face of the tip 50, and the
entire probe fixtured in a structure, not shown, which maintains
the position of tip 50 and provides a reference for gauging the
distance between the tip 50 and the surface 100.
Referring still to FIG. 2, the circuit schematically outlined there
operates by dividing the excitation supplied by the oscillator 90
into two capacitive arms 86 and 88. The diode matrix 74 formed as a
four diode path further controls the excitation flowing through
each capacitive arm 86 and 88 so that during a half cycle of a
given excitation polarity excitation is conducted from each arm 86
and 88 to separate tip 50 and 68 and, thence to ground via the
cylinder 40 and surface 100. During the following half cycle of the
opposite polarity the diode matrix 74 effects a switching of
excitation paths so that the excitation through each arm 86 and 88
is fed to the opposite tip of the tips 50 and 68 from the previous
half cycle. Over repeated cycles from the oscillator 90 the leads
82 and 84 will have a slight DC offset or average voltage and/or
current signal in addition to a large AC component. The amount of
offset signal indicates the capacitance difference between ground
and the tips 50 and 68. The AC is filtered out by the inductors 94
and only a DC signal is incident upon the meters 96 to give an
indication of capacitive unbalance between ground and the two tips
50 and 68.
Because the indicators 92 measure with respect to a zero reference
and only pass the offset component, the excitation of the
oscillator 90 may be made very large in order to increase the
sensitivity of the probe without the necessity for expensive and
accurate reference circuits.
As indicated above, the capacitive reactance of the arms or
capacitors 86 and 88 is usually at least an order of magnitude
smaller than the capacitive reactance between the cylinder 40 and
the tips 50 and 68. In this case, at any instant, the voltage
across the capacitors 86 and 88 will be very small and the voltage
on leads 72 and 76 connnected to the tips 50 and 68 substantially
equal to the voltage from oscillator 90. The average voltage on
leads 82 and 84 will be very close to ground or zero as compared to
the RMS value of the voltage from the oscillator 90. The resulting
near equality in the voltages on the tips 50 and 68 and their
structural relationships as indicated above, allows the tips to act
as capacitive shields or guards for each other and be closely
placed without interacting. An environmentally produced change
affecting one tip thus produces a similar effect upon both of the
capacitances between the cylinder 40 and tips 50 and 68 and
increases environmental independence.
By accurately machining the probe tip assemblies 44, further
unexpected improvements are achieved in the probe's immunity from
environmental influences operating to destroy the equality of the
capacitances between the cylinder 40 and the tips 50 and 68. To see
this more clearly, each probe tip assembly 44 must be viewed as a
coaxial capacitor in which the axial length of the capacitor is
short compared to the difference in radius of the inner and outer
conductor. In the usual coaxial capacitor, the capacitance is a
direct function of the axial length of the capacitor and an inverse
function of the logarythm of the ratio of the radii of the inner
and outer conductors. The fringe field is normally ignored. If the
inner and outer conductors and the dielectric material between them
are chosen to have equal coefficients of temperature expansion, a
temperature change for the coaxial capacitor will not alter the
ratio of the radii of the inner and outer conductors, but will
effect the axial length of the capacitor and therefore, the
capacitance of such a coaxial capacitor will vary directly with the
temperature coefficients of the material from which it is made.
As the height or axial length of the coaxial capacitor is decreased
until this length is only a small portion of the difference between
the radii of the inner and outer conductors, the axial length of
the capacitor is a less significant factor in its total
capacitance. The contribution of the fringe fields to the total
capacitance will be substantial. Thus, with small axial lengths the
ratio of the radii of the inner and outer conductors can be made a
more significant factor in the capacitances between the cylinder 40
and the probe tips 50 and 68. By selecting materials for the tips,
rings and annuli with equal temperature coefficients these
capacitances can be kept more nearly non-varying with temperature
changes.
By accurately machining the dimensions listed below, the probe tip
assemblies 44 can be made nearly identical over crucial dimensions
with the result that they have equal initial capacitance to a very
high accuracy. This equality then can be maintained over a large
temperate range because of the low temperature dependence indicated
above. There results an electronic capacitive probe of vastly
improved accuracy and environmental independence.
The dimensions which it has been found particularly important to
control are the following:
a. the circumference and the axial length of the periphery of the
tips 50 and 68 (axial length being less important when it is very
small);
b. the circumference and the axial length of the inner surfaces of
the rings 58 and 64; and
c. the spacing between the inner wall of the rings 58 and 64 and
the periphery of the tips 50 and 68 through the dielectric annuli
56 and 66 and the axial length of the annuli 56 and 66 to maintain
the spacing of the concentric capacitor formed between the rings
and tips.
With these dimensions kept to a tolerance of .+-. 0.0001 or .+-.
0.0002 inches a probe of great accuracy and sensitivity is
achievable by taking advantage of the theory and effects explained
above. Maintenance of these dimensional tolerances also insures a
high degree of repeatability in the sensitivity from one probe to
another. The sensitivity is defined as change in capacitance
between the first probe tip 50 and ground with respect to changes
in the distance between the surface 100 and the outer face of the
tip 50.
Other dimensions in the probe are less critical than those
mentioned above and may be held to normal machining tolerances
without a loss in the accuracy, environmental immunity, or other
performance characteristics of this probe. The guarding effect of
each tip 50 and 68 upon the other also helps to reduce the required
machining accuracy in the inner wall 46 and the spacing between
tips.
In deciding the radial distance between the outer peripheries of
the tips 50 and 68 and the inner surface of the rings 58 and 64, or
the radial thickness of the annuli 56 and 66 an important factor is
the expected maximum distance between the tip 50 and the surface
100 under the conditions of probe use. As this distance increases,
significant portions of the electrostatic flux between the outer
face of the tip 50, of the first assembly and points of ground
potential will run to the ring 58 instead of to the surface 100. A
corresponding reduction in the sensitivity of the probe is
experienced by this "flux switching". The design of the dielectric
annuli 56 and 66 can therefore be made with a view toward the
maximum expected distance between the probe tip 50 and the surface
100. A practical rule for the radial thickness of the annuli 56 and
66 is to make the thickness twice the maximum expected distance
between the tip 50 and the surface 100.
A few additional constraints on the general location of surfaces
must be observed:
1. The inner wall of the sleeve 62 is important as it effects the
fringe fields from the second probe tip assembly 44 and therefor it
should intercept the ring 64 well beyond the interface between the
ring 64 and the dielectric annulus 66;
2. In the event that the surface 100 to which the distance from the
probe is to be measured, is either ungrounded or non-conducting,
the cross-sectional combined areas of the ring 58 and cylinder 40
facing toward the surface 100 should be respectively one or two
orders of magnitude greater than the cross sectional area of the
tips 50 facing toward the surface 100. This latter constraint
establishes a much lower impedance from the grounded cylinder 40 to
the surface 100 than from the tip 50 under the conditions of an
ungrounded or non-conducting surface 100.
Another major constraint upon the construction of the probe and one
which is important to securing a high degree of accuracy in any
environment is that tips 50 and 68, the dielectric annuli 56 and
66, and the rings 58 and 64 possess a similar coefficient of
thermal expansion. One combination which satisfies this thermal
expansion requirement comprises KOVAR tips and rings and glass
dielectric annuli.
In a normal working environment for a probe of the design indicated
above, the hygroscopic properties of the material used for the
dielectric annuli 56 and 66 are of great significance. The
slightest absorption of water or other material from the
environment of the probe into the dielectric annuli 56 and 66 or
the interior volume 48 can alter the two capacitances between the
cylinder 40 and the tips 50 and 68 to an extent that destroys the
accuracies otherwise obtainable with a probe having the above
indicated construction.
To achieve accurate operation of the probe, the dielectric annuli
56 and 66 should be formed from a material having a very low index
of absorption of water or other foreign material which may be in
the environment of the probe.
The dielectric in the annuli 56 and 66 should also form an
impenetrable barrier against entry of water or foreign material
from the environment of the probe to the interior 48. The substance
forming the tips 50 and 68 and the rings 58 and 64 is normally a
metal which is inherently non-absorptive and impenetrable. The
ductile filler 60 helps to completely seal the interior 48 of the
probe from its environment.
Examples of substances for the annuli which fulfill both the
requirements for hygroscopic properties and temperature coefficient
of expansion are hard or soft glass, Al.sub.2 O.sub.3, and other
oxides of metals which are not semi-conducting (as metallic oxides
of beryllium, silicon or titanium). Particularly to be avoided are
most epoxy-type dielectrics or potting compounds, it having been
found from their use in the earlier probes of FIG. 1 construction
that absorption by and penetration through these dielectrics
destroys the probe's accuracy and value.
Alternately, or additionally, the coating 98 in FIG. 2 may be
applied across the open end 42 and first probe tip assembly 44 of
the probe. This coating is designed as a barrier against the
penetration of water or other foreign material from the environment
of the probe into the dielectric annuli 56 and 66 and the interior
48. The use of the coating 98 is desireable even in the case of
non-absorptive dielectric annuli 56 and 66 because some water or
foreign material may collect on the surfaces not only of the
dielectric annuli but of the conducting rings 58 and 64 and tips 50
and 68. Coverings which have been found to be particularly good for
this purpose are silicone organic compounds in general and titanium
dioxide (TiO.sub.2). Also oxides of the metals aluminum or
beryllium may be used.
Several variations of the probe described above have been
discovered which augment certain of the advantages mentioned,
improve the performance of the probe, and/or reduce its cost of
manufacture.
In FIG. 4 there is shown in cross-section the two probe tip
assemblies of a probe basically similar to that in FIG. 2 having an
external conducting housing 102 with a first probe tip assembly 104
composed of ring 106, dielectric annulus 108, and central
conducting tip 110; and having a curved exterior face 112. Such a
curved exterior face 112 is called for where a surface 114 from
which the distance to the probe tip 110 is to be measured
capacitively is itself curved over the portion facing the probe tip
110. In other applications, the curved face 112 can be used to give
greater significance to certain regions between the surface 114 and
tip 110 in the response of an indicator 92 than to other
regions.
Referring to FIG. 5, there is shown in a cross-section and partial
schematic diagram an easily manufactured probe insert 116
constrained within a cylindrical conducting housing 118.
The insert 116 comprising a first probe tip assembly 120 and a
second probe tip assembly 122 held in axially displaced parallel
relationship by a conducting cylindrical retaining band 124 is
designed for ease of construction as an assembly and for latter
easy insertion within the cylinder 118. The first probe tip
assembly 120 is composed of a first electrically conducting probe
tip 126 surrounded by and in contact with on its outer periphery a
first dielectric annulus 128 which in turn is surrounded by and in
contact with on its outer periphery a first electrically conducting
ring 130.
The second probe tip assembly 122 is constructed substantially
identically to the first probe tip assembly 120 and has a second
electrically conducting probe tip 132, second dielectric annulus
134, second electrically conducting ring 136. Holding the first and
second probe tip assemblies 120 and 122 parallel to each other and
displaced along a common axis 138 is the retaining band 124 which
has at both ends of its cylindrical extent a portion of increased
side diameter of sufficient depth to accept each probe tip assembly
flush with the end of the band 124 and in a close fit between the
inside wall of the band 124 and the outer periphery of each
electrically conducting ring 130 and 136.
The tips, annuli, rings and retaining band may be of any circular
square or rectangular cross-sectional shape as described above.
They are equally dimensioned between first and second probe tip
assemblies to the same tolerances indicated for the probe of FIG.
2.
The entire assembly 116 is designed to fit within the cylinder 118
to form the completed probe structure. The portion 140 of the
cylinder 118 within which the assembly 118 is inserted is machined
to have a relatively thin wall and further consists of an elastic
material such as stainless steel. The thin stainless steel portion
140 has sufficient elasticity to allow the entire assembly 116 to
expand the slight amount necessary during temperature expansion and
contraction of the assembly 116. The assembly 116 is constructed to
have a uniform coefficient of temperature expansion throughout its
constituent party by, for example, forming all electrically
conducting portions of KOVAR, and the dielectric annuli of
glass.
The other end of the cylinder 118 opposite the portion 140 has a
threading 142 designed so that the cylinder 118 may be secured to a
further terminal structure, now shown, for holding the entire probe
in a defined relationship with a fixture. Hermetically sealing the
end of the cylinder 118 having the thread 142 is a two contact
electrical connector 144 utilized for mating with a proper two
terminal connector, not shown, on the terminal structure.
Each of the two terminals in the connector 144 is connected via
leads 146 and 148 respectively to opposite junction points of a
diode matrix 150 comprising a continuous ring of four joined diodes
152 connected for conduction in a single direction. The other two
junction points between the diodes in the ring of the matrix 150
are connected respectively to the first and second electrically
conductive tips 126 and 132, as described above. The diode matrix
150 may be placed either between the connector 140 and the first
probe tip assembly 122 or between the two probe tip assemblies 120
and 122.
Referring to FIG. 6, there is shown in sectional view an end region
of a capacitive probe where first and second probe tip assemblies
154 and 156 are located between a conventional cylindrical
conducting housing 158. The first and second probe tip assemblies
154 and 156 are composed of first and second dielectric substrates
160 and 162 respectively upon which respective electrically
conducting central depositions or spots 164 and 166 are placed as a
substitute for the probe tips described above.
Surrounding and spaced apart from the depositions or spots 164 and
166 on the substrates 160 and 162 are electrically conducting
deposition bands 168 and 170 respectively. Each band 168 and 170 is
coaxial with a central axis of each spot 164 and 166 which axis is
in turn co-incident with a central axis 172 of the cylinder 158.
Each substrate 160 and 162 is placed perpendicular to the axis 172
within the cylinder 158.
In forming the bands 168 and 170 by deposition, the conductive
material deposited is allowed to deposit onto outer peripheries 174
and 176 of the substrates 160 and 162 respectively. With the
peripheral shape of each substrate equal to the cross sectional
shape of the cylinder 158, this additional deposition to the
peripheries 174 and 176 allows both probe tip assemblies to fit
tightly within the cylinder 158 and yield an electrical contact
between the cylinder 158 and each electrically conducting band 168
and 170.
Electrical leads 178 and 180 are attached in a conventional manner
to the depositions or spots 164 and 168 respectively and lead
through the substrates 160 and 162 and deposition or spot 166 to
circuitry as described above.
It is of course possible to place the first and second probe tip
assemblies 154 and 156 respectively within a retainer band like the
band 124 indicated in FIG. 5 before they are inserted into the
cylinder 158.
The advantages of this construction are that the conductive
elements forming the capacitance plates for the capacitance between
the cylinder 158 and the central depositions or spots 164 and 166
have practically no axial dimension and thus no significant axial
temperature expansion. Also the thermal expansion of each probe tip
assembly 154 and 156 is determined entirely by the substrates 160
and 162 which are entirely homogeneous, have the same coefficients
of thermal expansion throughout, and produce a completely uniform
temperature response.
The depositions may be formed from a variety of processes
including: 1. fluxed glass bonded metal suspension; 2. sputtering;
3. vacuum evaporation; and 4. electroformation. It can be
appreciated that these techniques will allow economic mass
production at reduced costs while maintaining the same high degree
of dimensional tolerances between the first and second probe tip
assemblies.
It is also possible to form the first probe tip assembly 154 with
the electrical conducting depositions facing inwardly instead of
outwardly as shown in FIG. 6. This will prevent damage to the
surface containing the conducting elements as well as incorporate
an environmental barrier in the first probe tip assembly 154 by
proper selection of a non-absorptive and impenetrable substrate
material, such as glass.
Referring to FIG. 7 a capacitive probe is shown of the
substrate-deposition type wherein a modification is shown in the
second probe tip assembly. In this case, a substrate 182 forming
the substrate for the second probe tip assembly is shown axially
perforated with a hole 184 covering a substantial portion of the
area which would otherwise have comprised the central deposition or
spot of the substrate. In this case, a central deposition 186 for
the substrate 182 is formed adjacent to the hole 184 and may
further comprise a deposition through the hole 184 and over onto
the opposing surface of the substrate 182. An electrically
conducting deposition band 188 may be formed as indicated for the
deposition band in FIG. 6, or, alternatively in the case where the
central deposition 186 is deposited on both sides of the substrate
182, a deposition for the band 188 is made on both sides of the
substrate 182 and is joined across an outer periphery 190 of the
substrate 182 so as to provide an electrical connection between the
band 188 and a cylindrical housing 192 for the probe tip
assembly.
The purpose of the hole 184 through the substrate 182 of the second
probe tip assembly is to allow the first probe assembly 154, and in
particular the central deposition 164 thereon to be affected by
more of the fringe field that affects the capacitance between the
housing 192 and the central deposition 186 of the second probe tip
assembly. This effect is further augmented when the first and
second probe tip assemblies are brought close together within the
cylindrical housing 192. With both central depositions having in
common a large component of the fringe field the tendency for
environmental changes of the capacitive probe to create
inequalities in the capacitance between the cylinder 192 and the
central depositions is further decreased.
Of course the hole 184 through the second probe tip assembly may be
used in constructions similar to those of FIGS. 2 and 5.
Referring now to FIG. 8 there is shown in cross-sectional view and
partial schematic diagram a further modification of the basic
capacitive probe. Here the cylindrical housing 194 for the probe is
composed of two cylindrical concentric electrical conductors, an
outer conductor 196 and an inner conductor 198, having coaxially
therebetween a cylindrical dielectric spacer 200.
First and second probe tip assemblies 202 are shown in FIG. 8 but
will not be described in detail except to indicate that they can be
of any of the above-described constructions.
Electrically conducting leads 204 and 206 lead from each probe tip
assembly as above indicated toward a diode matrix 208. In the
construction of FIG. 8, however, the diode matrix 208 may be
separated from the probe tip by a length of cable 209 with dual
outer and inner shields 210 and 212 connected to the outer and
inner cylindrical conductors 196 and 198 respectively and having
the leads 204 and 206 contained within them. The cable 209
containing the shields 210 and 212 terminates in a circuit housing
214 containing measurement circuitry 216 including a diode matrix
208, capacitor arms 86 and 88, inductors 94 and oscillator 90 as
described above. The shield 210 of the cable 209 allows the
circuitry 216 to be located remotely from the region of the
capacitive probe.
The circuit housing 214 is composed of two dielectrically spaced
electrical conductors, an outer conductor 218 and an inner
conductor 220, connected respectively to the outer and inner
shields 210 and 212. The outer conductor 218 is in effect a circuit
ground or common point and is desirable but not necessary to shield
the circuitry 216. The inner conductor 220 does act as a shield and
its presence is important for shielding the measuring circuitry
216.
The output of the oscillator 90 which feeds the capacitor arms 86
and 88 is also connected to the inner conductor 220 and in turn the
inner shield 212 and inner cylindrical conductor 198.
Alternatively a D. C. blocking capacitor 222 may be connected
between the oscillator 90 and the capacitor arms 86 and 88 and the
inner conductor 220.
The inner cylindrical conductor 198, inner shield 212, and inner
conductor 220 all function as a guard for the probe tip assemblies
202, the associated electrical leads 204 and 206, and the measuring
circuitry 216. By guarding the assemblies 202 in the construction
of FIG. 8 with a shield at substantially the same instantaneous
electric potential as the leads 204 and 206 and the probe tips 50
and 68 in the assemblies 202, the effect of capacitance between
ground potential and the leads 204 and 206 and tips 50 and 68 can
be kept very small despite long distances and cable 209 lengths
between the cylinder 194 and the circuit housing 214. The smaller
the capacitance between ground and the leads 204 and 206, the
higher is the sensitivity of the probe defined as the percent
change in capacitance between ground and the lead 206 and tip 50
for a given change in the distance between the front tip 50 and the
surface 100 from which distance is to be measured. Also
environmental dependence is reduced since there is less capacitance
to be affected.
A further advantage of the guarded capactive probe of FIG. 8 is
that the equal potential on the inner cylinder 198 directs the
electrical field flux from the measuring or front probe tip 50
toward the surface 100 and away from the outer cylinder 196. The
presence of such a guard allows the capacitive probe to maintain
its sensitivity at greater distances from the surface 100 than
without the guard.
In FIGS. 9 and 10, a sectional view is shown of a modified form of
a guarded capacitive probe. A conducting cylindrical housing 226
having a single conductor 226 is provided with each probe tip
assembly composed of a dielectric substrate 228 with an
electrically conducting central deposition 230 on the surface of
the substrate 228. A guard is provided by an electrically
conducting deposition band 232 surrounding the central deposition
230 on the substrate 228. As indicated by FIGS. 9 and 10 the
ultimate construction of the capacitive probe may be either with
the depositions 230 and 232 facing out from the interior of the
probe, as shown in FIG. 9, or in the reverse configuration with the
depositions facing in, as shown in FIG. 10. The construction of
FIG. 10 presents the added advantage of protecting the depositions
230 and 232.
In the configurations of FIGS. 9 and 10 the guarding effect of the
band 232 will be limited to directing the electric field from the
central deposition 230 toward a measuring surface placed at a
distance from the end of the capacitive probe. There will also be
some effect from the deposition 232 in guarding the field between
the cylindrical housing 226 and the central deposition 230 but to a
lesser extent than in the case of the construction of FIG. 8.
If it is desired to operate the capacitive probes of FIGS. 9 and 10
remotely from the circuitry 216 of FIG. 8 with a cable connection
therebetween, it will be necessary to provide a shielded cable 209
in which there is a connection from an inner shield 212 to the
guard bands 232 and with the electrical leads from the central
depositions 230 passing centrally through the shield of the cable
209.
The description now turns to consider the operation of circuitry
designed for use with the above probes to give an output indication
of the capacitance between the probe's measuring tip or electrode
and a surface. The structure of this circuitry has already been
described briefly above.
Referring in particular to FIG. 11 there is shown an impedance
measuring circuit or bridge 234 fed by an oscillator 90 and
supplying exitation to detecting and reference capacitances 236 and
238 respectively. These capacitances 236 and 238 will normally be
measuring and balancing capacitances respectively. Within the
impedance measuring circuit 234 excitation supplied from the
oscillator 90 is divided between impedances in arms 240 and 242
which impedances are normally characterized by having a DC open
circuit and a substantially lower AC impedance than capacitances
236 and 238 at the frequency of oscillator 90. The current flowing
through the impedances 240 and 242 is incident upon a diode matrix
244 which switches the current from the impedances 240 and 242
between the detecting and balancing reference capacitances 236 and
238, depending upon the polarity of the current from the oscillator
90.
In this manner the excitation through junction points 246 and 248,
where the impedances 240 and 242 respectively join the diode matrix
244, will produce a signal at points 246 and 248 (as well as 262
and 264) with a DC component representative of the differences in
capacitance between the detecting and reference capacitances 236
and 238. AC filtering inductors 250 and 252 join the junctions 246
and 248 respectively to meters 254 and 256 for providing a return
to ground for this DC component and for indicating the DC component
at junctions 246 and 248 without being masked by the substantially
higher AC component from oscillator 90.
It is significant that the instantaneous voltage across
capacitances 236 and 238 and oscillator 90 are approximately equal.
This minimizes the effect of inter-electrode capacitance and allows
each electrode in the probes described to be closely placed or
allows a multiplicity of electrodes described below. The guarded
construction of FIG. 8 can also be easily realized as a result. One
terminal of the source, measured capacitance, and output may be
grounded. Also substantial differences between the capacitances 236
and 238 are possible while the output indication still varies
linearly with capacitance 236.
Output terminals 258 and 260 are connected to the junction point
between each inductor 250 and 252 and meters 254 and 256
respectively. These output points 258 and 260 provide pick off
terminals for the D. C. component at junctions 246 and 248 for
further signal processing as explained below.
The detecting and reference capacitances 236 and 238 are indicated
in FIG. 13 outside the impedance measuring circuit 234 but joined
to it via leads 262 and 264 of the diode matrix 244. For
convenience, hereafter, the terminals 262 and 264 will be
considered as input terminals to the impedance measuring circuit or
bridge 234 while the terminals 258 and 264 will be considered as
output terminals of the circuit 234.
As can be seen from the above description in conjunction with FIG.
11, if both the detecting and reference capacitances 236 and 238
are equal the D. C. output on either output 258 or 260 will be
zero. For small differences between these capacitances 236 and 238
the D. C. signal on output points 258 and 260 will be proportional
to that difference.
For the probe of FIG. 2 that difference will be the capacitance
between the tip 50 in the first probe tip assembly and the surface
100 from which distance is being gauged. Such capacitance,
according to the standard formula for capacitance between parallel
plates will be inversely proportional to the distance between the
probe tip 50 and the surface 100 from which distance is being
gauged. The output signal is thus inversely proportional to the
distance between the tip 50 of the first probe tip assembly and the
remote surface 100.
FIG. 12 shows a modification of the FIG. 12 circuitry which
provides an ultimate output directly proportional to distance being
gauged instead of inversely proportional. A bridge 266 is identical
to the impedance measuring circuit 234 but is instead excited by a
signal controlled oscillator 268 whose controlling signal is
supplied by a high regulator 270. The regulator 270 has
differentially inputted to it a reference 272 and an output of the
bridge 266 to form a negative feedback loop so that the excitation
of oscillator 268 is controlled in a way which holds the output of
the bridge 266 substantially equal to the reference 272.
The oscillator 268 also excites a bridge 274 which is identical to
the bridge 266. .varies. bridge 266 has detecting and reference
capacitances 236 and 238 inputted to it. C.sub.A is defined as the
difference between these capacitances. The bridge 274 has a set of
detecting and reference capacitances 276 and 278 input to it.
Defining C.sub.B as the difference in capacitance between
capacitances 276 and 278, I as the reference 272, F as the
frequency of oscillator 268, V as the voltage output of oscillator
268 and E as an output of bridge 274, the following equations
apply:
VF C.sub.A .varies. I
VF C.sub.B .varies. E
E .varies. IC.sub.B /C.sub.A
E is thus proportional to the distance being measured or the
reciprocal of the difference in detecting and reference
capacitances from bridge 274.
Referring to FIGS. 13 and 14 there is shown respectively a vertical
section and horizontal cross section of a self-compensating
multiple probe tip capacitive probe having a rectangular
electrically conducting cylindrical housing 280 enclosing first and
second probe tip assemblies 282 and 284 respectively. Each probe
tip assembly is substantially identical and consists of a
dielectric sheet 286 having two plane parallel faces perforated
with three holes which are in line from left to right across FIGS.
13 and 14 and pass perpendicularly between surfaces of the sheets
286. Into each hole an electrically conducting probe tip or
electrode 288 is inserted and made flush with the surfaces of the
dielectric sheet 286. A small amount of a ductile filler as
mentioned above may be used between the dielectric sheet and the
periphery of the electrically conducting tips 288.
An open end 290 of the cylindrical housing 280 has the first probe
tip assembly 282 mounted flush with it and perpendicular to a
central axis 292 of the cylindrical housing 280. The second probe
tip sssembly 284 is mounted parallel to the first assembly 282 and
back from the open end 290 within cylindrical housing 280. An
exterior surface 294 of the first probe tip assembly 282 faces an
electrically conducting plate 296 having a surface 298
substantially parallel to the exterior surface 292 of the first
probe tip assembly 282 to define a path between the surfaces 298
and 294 through which a strip or band of dielectric material 300
can pass in a direction perpendicular to the drawing surface of
FIG. 13.
The measuring tips or electrodes in the first probe tip assembly
282 should be positioned with regard to the dimensions of the
dielectric strip 300 and in particular to its left and right hand
edges 302 and 304 respectively (as shown in FIG. 13) so that the
edges 302 and 304 will fall between the plate surface 298 and left
and right electrically conducting measuring probe tips 306 and 308
respectively. A central electrically conducting measuring probe tip
310, intermediate the tips 306 and 308 will then have between it
and the plate surface 298 a continuous portion of the dielectric
strip 300.
Inserted in the dielectric sheet 286 of the second probe tip
assembly 282 are the electrically conducting balancing probe tips
312, 314 and 316 which correspond in position in the second probe
tip assembly 282 to the electrically conducting measuring probe
tips 306, 308 and 310 in the first probe tip assembly. In this
manner three electrode sets of balancing and measuring tips or
electrodes are defined, each composed of a measuring and balancing
tip or electrode from corresponding positions in the first and
second probe tip assemblies 282 and 284. Electrical leads 318 are
connected, one to each electrically conducting tip or electrode,
and are lead through the capacitive probe away from the plate 296
to circuitry to be described below.
Though the self-compensating multiple tip probe as shown in FIGS.
13 and 14 has all electrode sets within a single housing, it is
possible for several single electrode set housings like that in
FIG. 2 to be fixtured together to produce a multi-tip probe with a
path between a plate surface 252 and a surface of the probe which
is a composite of several measuring electrodes from the several
separate housings.
For high measuring accuracy in the probe of FIGS. 13 and 14 certain
critical dimensions must be held to high tolerances (on the order
of .+-.0.0001 inches) while others can be kept to general machining
tolerances (.+-.0.005 inches or better). The critical dimensions
are:
1. Equality between tips of the same set in the peripheral axial
thickness and area of the perimeter edge of each tip or electrode
that faces the housing 280; and
2. Equality between tips of the same set in the spacing through the
dielectric sheets 286 and their axial thickness to the housing
280.
Because all tips will be operated at approximately the same
electric potential, the spacing between them is less critical.
The multiple tip capacitive probe of FIGS. 13 and 14 is
particularly suitable for gauging variations in the edge 302 to
edge 304 width of the dielectric strip 300 with compensation for
variations in the thickness and/or dielectric constant of the strip
300. This can be accomplished by detecting variations in the
capacitance between the plate surface 298 and the tips 306 and 308
which will be indicative of both width and thickness variations of
the dielectric strip 300. Variations in the capacitance due to
fluctuation in the thickness or dielectric constant of the strip
300 can then be compensated for by variations in the capacitance
between the plate surface 298 and the intermediate tip 310 since
these variations will be the result only of thickness fluctuations
in the dielectric strip 300.
Where the distance between the probe and surface is likely to vary,
an addition to the probe of FIGS. 13 and 14 is required to provide
compensation for the variation. This modified probe is shown in
FIG. 15 in a diagrammatical and sectional elevation view. In the
FIG. 15 construction two additional electrode sets composed of
measuring electrodes or tips 320 and 322 and balancing tips 324 and
325 are placed either side of the three electrode sets of FIGS. 13
and 14. The surface 298 of the plate 296 must extend so that all
five electrode sets face it. The measuring tips 306,308 and 310 are
positioned relative to the dielectric strip 300 in the same way as
indicated for FIGS. 13 and 14. The measuring tips 320 and 322 then
face the surface 298 without any portion of the strip 300
intervening.
It is here convenient to indicate the formulas giving the
capacitance between the surface 298 and the measuring tips 306,
308, 310, 320 and 322 in terms of the separation and properties of
the intervening strip 300. These capacitances, which are C.sub.306,
C.sub.308, C.sub.310, C.sub.320, and C.sub.322 respectively, are
given as follows: ##SPC1##
C.sub.p = C.sub.320 = C.sub.322 = alk.sub.1 /D
where:
a = width of each measuring tip;
D = distance from the measuring tips to the surface 298;
k.sub.1 = permittivity of the probe environment
k.sub.2 = permittivity of the dielectric strip 300;
l = length of each measuring tip;
p.sub.1 = the amount of the tip 306 that the strip 300 extends
below in the a direction;
p.sub.2 = the amount of the tip 308 that the strip 300 extends
below in the a direction; and
t = the thickness of the dielectric strip 300.
Turning now to consider systems which accomplish the gauging of the
width of a dielectric strip 300 using the apparatus of FIGS. 13, 14
and 15 and the circuity of FIG. 11 it can be seen that if the
detecting capacitance 236 in the FIG. 11 is a parallel combination
of the capacitance between ground and the electrically conducting
tips 306, 308 and 316 while the reference capacitance 238 is a
parallel combination of the capacitance between ground and the
electrically conducting tips 312, 314 and 310, the output at either
point 258 or 260 will be representative of variations in the width
and thickness and dielectric constant of the dielectric strip 300
as the strip 300 is passed between the probe and the plate surface
298.
Specifically, the output at either point 258 or 260 will be
proportional to C.sub.D - C.sub.R as given above. The derivative
with respect to t is:
This can be made zero if a = p.sub.1 + p.sub.2 at a nominal width
for strip 300 and will remain very close to zero for small width
variations or variations in p.sub.1 + p.sub.2 from a nominal strip
300 width. Under these circumstances the outputs of points 258 and
260 of the impedance measuring circuitry 234 of FIG. 11 will vary
from zero with and only with variations in the width of the strip
300 shown in FIGS. 14 and 15.
Turning now to the block diagrams and partial schematics of FIGS.
16 and 17 means are shown for gauging the width and thickness of
the dielectric strip 300 using a capacitive probe of the type shown
in FIGS. 13, 14 or 15.
In FIG. 16, a series of paired electrode sets 326, 328, 329, 330
and 331 represent respectively the measuring and balancing
capacitances to ground from each set of electrodes or tips 306 and
312; 308 and 314; 310 and 316; 320 and 324; and 322 and 325 in FIG.
15. The ungrounded side of these paired capacitances 326, 328, 329,
330, and 331 are connected to input terminals of bridge circuits
332, 334, 336 and 337 respectively with an electrical connection
between each measuring electrode and between each balancing
electrode of the sets 330 and 331 so that the parallel combination
from these sets is inputted to bridge 237. The bridge circuits 332,
334, 336 and 337 are typically duplicates of the impedance
measuring circuit shown in FIG. 11, and their inputs correspond to
the inputs 262 and 264.
An output of bridge 332 is fed to an input of a summer amplifier
338. Fed into a second input of summer 338 is an output of the
bridge 334. Through proper interconnection between the paired
capacitances 326 and 328 and inputs 262 and 264 of the bridges 332
and 334 and proper selection of the outputs 258 and 260 of these
bridges the output of the summer 338 can be made equal to c.sub.D
as given in Equation (2) above. This can be accomplished by
connecting the tips 312 and 314 to the 262 inputs and the tips 306
and 308 to the 264 inputs and by using the 258 outputs of the
bridges 332 and 334. Alternatively bridge 334 could be eliminated
with both tips 312 and 314 connected to the 262 input of bridge 332
and with both tips 306 and 308 connected to the 264 input as
similarly done with bridge 337 and sets 330 and 331. The output of
summer 338 is then replaced by the output of bridge 332.
A voltage controlled oscillator 340 outputs an alternating electric
potential fed to the bridges 332, 334, 336 and 337 as excitation.
The excitation VF, of the output of the oscillator 340 is
controlled by the signal input to it from a regulator 342. The
output signal of summer 338 (or alternatively of bridge 332) to a
terminal 344 is thus proportional to F C.sub.D where C.sub.D is
given by Equation (2). The output of signal of bridge 336 to a
terminal 346 is proportional to FC.sub.R from Equation (1) and the
output signal of bridge 337 to terminal 348 is proportional to F
C.sub.p from Equation (3).
The terminals 344, 346, and 348 may be connected to various
terminals of analog or digital logic circuitry to process these
signals to yield a signal representative of the width of the strip
300 without variations due to changes in k.sub.2, t or D, the strip
permittivity, thickness or distance between probe and surface
respectively. The functional elements of this logic circuitry will
now be described in conjunction with FIG. 16.
Then FIGS. 17a-17c indicate various interconnection schemes between
this logic circuitry and the terminals 344, 346, and 348.
The regulator 342 controlling the oscillator 340 has input
terminals 350 and 352 which receive a signal dependent on the
frequency of oscillator 340. The regulator 342 adjusts this
frequency until the signals on terminals 350 and 352 are
substantially equal.
A bridge 354, substantially the same as bridge circuit 234 and
excited by oscillator 340, has fixed capacitances 356 and 358
between ground and the inputs 262 and 264 of the bridge 354. The
capacitances 356 and 358 are of a value approximately the same as
nominal capacitances in the sets 326, 328, 329, 330, and 331, but
differ by a fixed amount defined as C.sub.F. The 258 output of
bridge 354 is differentially inputted to a differential amplifier
360 along with a constant 362. The output of amplifier 360 is fed
to a denominator input of a divider circuit 364 which outputs to a
terminal 366 the quotient of a signal on a numerator input to the
divider 364 from a terminal 368 divided by the signal at its
denominator input.
A differential amplifier 370 is also provided in the logic
circuitry and has two differential inputs connected to terminals
372 and 374 with an output of the amplifier 370 conducted to a
terminal 376. A further divider circuit 378 operates like the
divider circuit 364 and has its numerator input, denominator input,
and output connected to terminals 380, 382, and 384 respectively. A
differential amplifier 386 has two differential inputs with the
non-negating input connected to a terminal 388, the negating input
connected to a constant signal source 390, and the output connected
to a terminal 392. Finally a source of a constant reference signal
394 has that reference signal, I, connected to a terminal 396.
In FIG. 17a, the first alternative interconnect system among the
terminals of FIG. 16 is shown with the terminals connected as
follows: 344 to 374; 346 to 350; 396 to 352 and 372; and 376 to
388.
With the configuration of FIG. 17a the signal at terminal 346, VF
C.sub.R, is kept equal to I at terminal 396 by the regulator 342.
Thus,
VF = I/C.sub.R (4)
The signal at terminal 344 is FC.sub.D and after substituting for F
from Equation (4) is IC.sub.D /C.sub.R . Terminal 376 after
amplifier 370 then has I (C.sub.D - C.sub.R)/C.sub.R (5)
This can be expressed in terms of the physical dimensions of the
probe as
Assuming that D can be kept constant by proper fixturing of the
probe of FIG. 15 relative to the surface 298, equation 6 can be
made independent of variations in t at a nominal value of p.sub.1 +
p.sub.2 by designing the probe for a = p.sub.1 + p.sub.2. In this
way variations in t affect only the sensitivity of the system and
not the existence of a deviation from the nominal value for p.sub.1
+ p.sub.2.
FInally amplifier 386 and constant 390 allow scaling of Equation
(6) and the addition of an offset respectively so that the output,
Eo, at terminal 392 may be made equal to the width of strip
300.
In FIG. 17b the terminals are interconnected as follows: 344 to
374; 346 to 350; 368 to 376; 396 to 352 and 372; and 366 to
388.
The circuit operation produced by the connections in FIG. 17b
follow those in FIG. 17a except that terminal 376, the output of
amplifier 370 corresponding to Equation (6), is fed to terminal
368, the numerator input of divider 364 for division by the output
of amplifier 360 on the denominator input.
The output of bridge 354 is
If the gain of amplifier 360 is a 1 k.sub.1 k.sub.2 and the value
of constant 362 is (D k.sub.2)/(a l k.sub.1 k.sub.2) the output of
amplifier 360 becomes t (k.sub.1 - k.sub.2). Divider 364 then
produces as its output a signal with the value:
which varies directly with the width of the strip 300. Amplifier
386 and constant 390 then operate the same as in FIG. 17a to output
at terminal 392 a signal which may be made equal to the width of
strip 300 or its deviation from a preselected value.
In FIG. 17b it has been assumed that the distance D between probe
and surface does not vary significantly. FIG. 17c shows an
arrangement which allows D to be eliminated as a factor in the
output of the logic circuitry when D varies.
In FIG. 17c the terminal connections are: 344 to 374; 346 to 350;
348 to 382; 368 to 376; 366 to 388; 396 to 352 and 372; and 380 to
392. The operation is similar to FIG. 17b except that the output of
amplifier 386 is divided by C.sub.p from Equation (3) present at
terminal 348 to eliminate D from Equation (8).
Referring now to FIG. 18, a further example is shown of a system
for use in conjunction with the width measuring gauge of FIGS. 13
and 14 which system will provide an output varying directly with
the width of the dielectric strip 300 where D is constant. Shown in
FIG. 18 are two pairs or sets of measuring and balancing
capacitances 400 and 402. Each capacitance in the pair 400 is a
parallel combination of the respective balancing and measuring
capacitances between ground and the outside electrically conducting
tips in FIG. 13. (i. e., tips 306 and 308 are electrically joined
and tips 312 and 314 are joined.) The pair 400 feeds a bridge 404
in a manner such that the output of the bridge 400 as amplified by
an amplifier 406 connected on the output of bridge 404 is equal to
C.sub.D from Equation (2). The pair 402 with leads from tips 310
and 316 feeds a bridge 408 which in turn through an output terminal
feeds amplifier 410 to yield at the output of the amplifier 410 a
signal equal to C.sub.R from Equation (2). These signals are
differentially combined in an amplifier 412 to produce an output
equal to (C.sub.D -C.sub.R). This output of amplifier 412 is fed to
a divider 414 on a numerator input. The denominator input of the
divider 414 is connected to the output of amplifier 410. The
output, E.sub.D, of the divider 414 is consequently (C.sub.1 +
C.sub.3 - C.sub.2 )/(C.sub.2).
In order to eliminate variations in the signal E.sub.o due to
variations in the thickness, t, of the dielectric strip 300 an
inverter 416 is fed by the output of amplifier 410 and the inverter
382 outputs a signal equal to 1/ C.sub.R. An amplifier 418 of gain
1 a k.sub.1 k.sub.2 is fed differentially the output of inverter
416 and a constant 420 equal to D 1 a k.sub.1 will output a signal
equal to the variable t (k.sub.1 - k.sub.2). An additional divider
422 is fed E.sub.o on a numerator input and the output of amplifier
418 on a denominator input. The output of the divider 422 will be
equal to
The
output of the divider 422 is then fed to an amplifier 424 as one
input with a constant 426 added differentially to a second input of
the amplifier 424. The output will be a signal equal to the width
of the dielectric strip 300 after suitably adjusting the gain of
the amplifier 424 and the value of the constant 426 as indicated
above for amplifier 386 and constant 390 in FIG. 16.
* * * * *